While the technology for the construction of tunnels for various end used is quite old and well-developed, it has received new emphasis as a result of tunneling requirements for rapid transit systems. The rapid transit systems in the Oakland-San Francisco Bay Area, Washington, D. C. and planned systems in areas such as Atlanta have all made or proposed to make widespread use of long tunnels underneath the urban and densely populated areas.
While such tunnels can be constructed and permanently supported by means of steel tunnel support structures, it has been demonstrated that reinforced concreate tunnels can be advantageously employed during construction and as permanent installations. The use of such reinforced concrete tunnels results in a substantial cost savings, and cost over-runs have plagued all of the rapid transit systems installed to date.
One of the problems connected with the use of concrete tunnel support structures is the manner by which the concrete tunnel sections are coupled together. The reinforced concrete tunnel is formed from a plurality of cylindrical sections which are coupled together to form a unit. In a typical installation each tunnel section might be 16 feet in diameter and 4 feet in axial length, and the sections must be effectively and efficiently coupled together as a single unit. For the purpose of ease of construction, each section is further broken down into segments which are initially brought into the tunnel as a portion of a cylinder, for example, one quarter of the cylindrical section, and then are assembled.
Attempts to simply bolt the concrete sections together have encountered substantial problems. Even when the tunnel is straight, the tunnel sections can become slightly misaligned, making it most difficult and tedious, and sometimes impossible, to use conventional coupling techniques, such as a bolt-together system. Additionally, most rapid transit tunnels have the further requirement that the tunnels must, on occasion, be curved. Such curves are not extreme, but typically on the order of one or two degrees in the axial misalignment between the adjacent tunnel sections which are 4 feet in length. This permanent misalignment is far in excess of what can be accommodated by conventional bolt-together systems.